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Home >> Mazda >> 2003 >> Protege5 Automatic >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Self-Diagnostics >> Diagnostic Tests >> DTC P0421: Catalyst System, Efficiency Below Threshold >> Diagnostic Procedure

Diagnostic Procedure

  1. Verify Related Repair Information Availability -  Turn ignition off. Connect scan tool to DLC-2. Turn ignition on, engine off. Monitor for FREEZE FRAME DATA and ensure data has been recorded. Check service bulletins for repair information related to this DTC. If repair information is not available, go to next step. If repair information is available, perform diagnosis and repair as necessary. After repair, go to step  6.
  2. Verify Related Pending Code & Stored DTCs -  Turn ignition off, then start engine. Using scan tool, check for pending code related DTCs. See RETRIEVING DIAGNOSTIC TROUBLE CODES  under SELF-DIAGNOSTIC SYSTEM. If no other DTCs are present, go to next step. If any other DTCs are present, go to appropriate trouble shooting procedure. See DIAGNOSTIC TROUBLE CODE DEFINITIONS .
  3. Inspect Exhaust System Gas Leakage -  Inspect exhaust system for any leakage. If exhaust system is okay, go to next step. If any exhaust system leak is present, repair or replace faulty component. After repair, go to step  6.
  4. Inspect Installation Of Front & Rear Oxygen Sensors -  Ensure both HO2Ss are installed correctly (tight). If no problem is found, go to next step. If a loose HO2S is found, retighten HO2S(s). After repair, go to step  6.
  5. Inspect WU-TWC Converter -  Check operation of WU-TWC converter. See WARM UP THREE-WAY CATALYTIC CONVERTER under EMISSION SYSTEMS & SUB-SYSTEMS in SYSTEM & COMPONENT TESTING - MAZDA6, MIATA, MPV, PROTEGE & PROTEGE5 article. If WU-TWC converter is okay, replace suspect HO2S. After repair, go to next step. If WU-TWC converter has deteriorated, replace WU-TWC converter. After repair, go to next step.
  6. Verify Trouble Shooting Of DTC P0421 Completed -  Reconnect all connectors. Turn ignition on, engine off. Clear DTCs. See CLEARING DIAGNOSTIC TROUBLE CODES  under SELF-DIAGNOSTIC SYSTEM. Using scan tool, perform MODE 1 - PCM ADAPTIVE MEMORY PROCEDURE DRIVE MODE , MODE 2 - EGR SYSTEM REPAIR VERIFICATION DRIVE MODE  and MODE 3 - HO2S, HO2S HEATER & TWC REPAIR VERIFICATION DRIVE MODE  under SELF-DIAGNOSTIC SYSTEM. Check for DTCs. See RETRIEVING DIAGNOSTIC TROUBLE CODES  under SELF-DIAGNOSTIC SYSTEM. If DTC P0421 is present, replace PCM. For PCM removal and installation, see POWERTRAIN CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION article. After repair, go to next step. If same DTC P0421 is not present, go to next step.
  7. Verify After Repair Procedure -  Using scan tool, perform After Repair Procedure. See AFTER REPAIR PROCEDURE . If no other DTCs are present, testing is complete. If any other DTC is present, follow appropriate DTC diagnostic procedure. See DIAGNOSTIC TROUBLE CODE DEFINITIONS .
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